This invention relates to a method and apparatus for the detection, identification and measuring of the number and sizes of particles on a material, such as the surface of a semiconductor wafer.
In the prior art, there have been a variety of ways in which to detect, and measure the number and sizes of particles on a semiconductor wafer, for the purpose of rejecting those wafers which had on the surface, one or more particles of undesired sizes, e.g. from 1 to 20 microns, or for those having on the surface an excessive number of such particles. One of the most prevalent methods employes the human operator using a light field/dark field microscope. Using the human eye, the human operator would actually count the number of particles and also identify the size of the particles, such as those between 1 to 20 microns, and then remove those wafers which had an excessive number of particles or those having particles of a certain size. This method is without doubt highly inaccurate, and very expensive both in terms of wages for the human operator, and in terms of the number of rejects both after the inspection and after production of the chips (when the erroneously passed wafer would be found to have an electrical defect, e.g. short circuits, because of the presence of contaminant particles).
It is well known that contaminant particles on a semiconductor surface can cause open circuits, short circuits and other defects in the integrated circuitry placed thereon or manufactured thereof. Integrated circuits can have circuitry as small as 2.5 microns or less in more advanced technology. For the sake of economy, and since there are so many steps for the manufacture of integrated circuits after inspection of the raw wafer surface, the contaminated wafers must be removed from the production line before the numerous steps of manufacture are started. In some cases, wafers having an excessive number of particles about 1 micron in size must be removed, although at the present state of the art, the particles can be slightly larger in many cases and still be acceptable for circuitry preparation.
Another method is the use of a helium/neon laser to scan the wafer surface and using a photomultiplier to detect the reflected beams. The particles on the wafer surface will reflect light from the laser beam unto a photomultiplier with the number of pulses received by the photomultiplier being the particle count, and the intensity of the pulses being the indication of the size of the particles. This method, however, has not been fully exploited and developed, and has such defects as requirement of an expensive set of optical devices to form the appropriate laser beam and to scan same across the wafer surface, and requirement of mechanical masks to direct the beam. Also, the scanning, whether done by a set of optical devices or otherwise, is slow, requiring about 2 to 4 seconds for a 3 inch wafer. The photomultiplier detector also can scan only one spot at a time as the laser spot is removed across the wafer surface. Thus, there is an inherent problem with the use of laser beams, namely, that only one spot at a time is detected. The laser beam is not scanned at a very fast speed across the wafer surface. Also, disadvantageously, the photomultiplier cannot detect which particle is moving and which is stationary since the light used to illuminate the particles is the narrow beam of the laser and the moving particles which are hit by the laser beam on one line scan may be missed on the next line scan. Thus, there is no way in which an accurate map could be made of the moving particle, and hence, such moving particle may be viewed by the photomultiplier as being a stationary particle. There is no way that the laser beam can be scanned across the wafer and down the wafer at the same speed as a vidicon electron gun which scans a television camera. A further and more practical disadvantage to use of a laser system is the expensive auxiliary safety equipment which must be employed in conformance with governmental standards and regulations for the use of lasers.
There is an urgent need, for example, in the semiconductor industry, as well as other industries, for a simple and inexpensive method and apparatus for the detection, identification and measurement of the number and sizes of particles on the surface of or in a material, such as a semiconductor wafer, which also can be operated by an unskilled operator. The particles to be detected are from 0.3 microns and above. Particles larger than 20 microns will usually be priorly removed by ordinary cleaning methods.